The Moho Architecture and Its Role for Isostasy-Insights From the Lofoten-Vesteralen Rifted Margin, Norway

The crustal structure of the Nordland and Troms region, Norway, has received growing scientific attention because (a) the region is one of the most seismically active areas of mainland Norway, and (b) there are differing interpretations of the crustal structure but none of the proposed models simultaneously satisfy gravity, topography and crustal isostasy. At the core of the puzzle is the Lofoten-Vesteralen archipelago, which exhibits considerable variations in crustal thickness, seemingly inconsistent with the topographic expression along this geomorphic structure. The prevalent view has been that the crust beneath the southern Lofoten is extremely thin (similar to 20 km). This has recently been disputed. Here, we address this debate by producing new lithospheric models in the region from joint inversion of receiver functions and P-wave polarizations at 62 seismic stations. Our results are consistent with the regional trends from other models, including a shallow Moho in the southern Lofoten. Moreover, our results detect a low-velocity layer in the uppermost mantle, which appears to be highly relevant to isostasy in the region. We conclude that the crustal structure in the region may not be as controversial as the recent debate suggested. What appears more urgent to understand is how the concept of isostasy is defined, and how it relates to the layered structure of the lithosphere. In particular, our findings emphasize the importance of conceptualizing the Moho as a transition zone with considerable thickness and internal structural variations, rather than a simple velocity discontinuity.